A pressure reducing valve keeps an output pressure (secondary pressure) constant in the case of a variable inlet pressure (primary pressure).
Such a pressure reducing valve is disclosed in printed document DE 35 37 336 A1. This has a switching valve as a main stage and a control valve as a pilot stage. By means of the control valve, the secondary pressure is controlled up to a specified pressure limit. If the secondary pressure exceeds the pressure limit, then a spool of the switching valve is operated in such a way that a control-oil port is connected directly to an output port and the connection is no longer carried out via the control valve. A disadvantage in the case of this solution is that the entire pressure reducing valve is prone to vibrations. During the control process in the pilot stage, two restrictors of the control valve are opened in turn. Especially the opening and closing of that restrictor which controls the opening cross section between the control-oil port and the output port leads to a vibration excitation of a spool of the control valve of the main stage.
Further pressure reducing valves are disclosed in printed document DE 195 26 601 B4 and in printed document DE 37 39 824 C2.